1. Field of the Invention
Example embodiments of the present invention relate to off-axis projection optics and an extreme ultraviolet (EUV) lithography apparatus employing the same.
2. Description of the Related Art
One conventional exposure technique used for sub-100 nm direct-write photolithography in a semiconductor fabrication process involves using an exposure wavelength in an EUV region. EUV lithography uses EUV with a wavelength shorter than 100 nm, for example, a short wavelength of about 13.5 nm.
In an EUV region, because most materials have high light absorption, a refractive optical element may not be used. Thus, an exposure technique using EUV may require a reflective mask and projection optics including multiple reflective mirrors that may allow EUV light reflected from the reflective mask to proceed toward a wafer. For example, EUV light may be irradiated on a reflective mask disposed within a chamber and light reflected from the reflective mask may be reflected by reflective mirrors in projection optics and then may be incident onto a wafer to form a pattern corresponding to the reflective mask on the wafer.
In EUV lithography requiring projection optics including multiple reflective mirrors as described above, it may be difficult to use on-axis projection optics because EUV light proceeds by reflection.
Conventionally, use of a projection optics employing on-axis optics with mirrors sharing a common optical axis has been proposed. However, the proposed projection optics using on-axis optics requires a large number of mirrors (e.g., 6 mirrors) because an aberration introduced by each mirror is corrected while maintaining an on-axis optical system. This is because a structure using only on-axis optics cannot meet a requirement that an incident EUV beam be symmetrical about a central axis of a mirror in order to reduce aberration from the mirror. Herein, a structure using on-axis optics refers to a structure in which mirrors used correspond to portions of mirrors constituting the on-axis optics.
An increased number of mirrors in conventional projection optics using on-axis optics may increase the manufacturing costs and may reduce overall reflectance. That is, because reflectance in an EUV region is significantly lower than that in a visible light region, an increased number of mirrors may significantly reduce the overall reflectance of projection optics and thereby light utilization efficiency. As is known in the art, an amount of EUV light decreases by about 35% each time the EUV light passes through a mirror. For example, if a number of mirrors used is six and a reflectance of each mirror is 65%, the overall reflectance is about 7% ((0.656)×100%). That is, only about 7% of the original amount of EUV light remains after it passes through the six mirrors.